3-D printing usually calls to mind hobbyists and garage tinkerers, but the latest innovations in the field could have major health implications. Last year, doctors transplanted the world’s first 3-D printed ribcage. And recently, the Wake Forest School of Medicine in North Carolina announced its ability to print human muscles and tissues that could one day replace real organs.

Similar to an inkjet, the machines use blueprints to produce three-dimensional objects, layer by layer, using a computerized nozzle. But instead of layering molten plastic or metal, the Wake Forest team of bioengineers devised a printing method that allows them to create bone, cartilage, and muscle tissue. They start with scans of human ears, jawbones, and muscles, then print their prototypes using nozzles that dispense a mixture of polymers, living cells, and nutrients the research team calls “bioink.” The resulting organs are then implanted into mice and rats.

The concept is not a new one. Scientists have long been using bioprinters to create pieces of bone or organ. But until now, they have not been able to keep tissue cells alive. For a host to successfully adopt a new body part, new organs need to be fed with tiny capillaries. Because blood vessels are too delicate and tiny to 3-D print like the rest of the organ, the Wake Forest team left small channels in the tissue for blood vessels to develop. Figuring out how to develop this constellation of microchannels was the answer.

“You’re basically creating a vascular network with a printer,” said Dr. Anthony Atala, director of the Wake Forest Institute for Regenerative Medicine in an interview with reporter Rebecca Robbins.

The Wake Forest team is one of many in mostly academic labs around the globe using molds to create a vascular system for 3-D-printed tissue.

Other researchers have demonstrated vascularization techniques like seeding channels with the cells that line the inside of blood vessels, or connecting implants directly in line with a rat’s artery. Atala and his Wake Forest colleagues tried a different tack: relying on the host animal’s healing systems to naturally fill the empty channels with blood vessels.

Figuring out how to pattern the rivers of microchannels is key to getting blood to flow properly between the layers of 3-D-printed cells because, without a blood supply, the dense layers of tissue found in complex organs won’t survive in any configuration that’s larger than about the thickness of a sheet of paper. That’s been “the basic limitation of this field forever up to this point,” Atala said.

And did the ear survive? Within a month of implanting the 3-D ear into its rodent host, blood vessels had affixed themselves to the implant.

Research is a long ways off from creating replacement organs that can be transplanted into humans, but Atala and his team are hopeful.